As is widely known, an IP network, a representative example of which is the Internet, is basically a best-effort network. A best-effort network is one in which, although a best effort is made to achieve communication quality, there is no guarantee of quality. In such as best-effort network, because there is not necessarily a guarantee that a transmitted packet will be delivered, the time of delivery and timing interval of delivery are not guaranteed. However, because services such as voice and video communication require real-time performance, it is necessary to guarantee the delivery time and delivery time interval of packets to some degree. Real-time performance is performance in which there is almost no transfer delay (latency). For this reason, in an IP network, communication quality control technology called QoS (quality of service) is used to raise the priority level or maintain a certain communication speed of particular traffic in certain communication, while being based on the above-described best-effort communication.
In this case, typical forms of the above-noted QoS technology include DiffServ (differentiated services) disclosed in Non-Patent Reference 1 noted below and RSVP (Resource Reservation Protocol) disclosed in Non-Patent Reference 2 noted below. The above-noted DiffServ is a technology that controls the priority handling of packets, in accordance with a value stored in a particular field of the packet (for example, the DSCP (Differentiated Service Code Point) in the header of a packet conforming to IPv6). The above-noted RSVP is technology for guaranteeing quality on a communication path in an IP network, by reserving bandwidth up until the transmission destination in the IP network and by transmitting a packet using the reserved bandwidth.
Patent Reference 1 noted below discloses technology for reducing time jitter in IP packet forwarding using such QoS technology.
The above-described QoS technology is not restricted to the Internet, and is used in networks requiring real-time performance. It is used, for example, in a network that forms a part of a distributed control system (DCS) implemented in a plant or factory. In a distributed control system, on-site devices (measuring instruments and actuators) known as field devices are connected with a management device managing and controlling the on-site devices, via a cable or wireless network. In this distributed control system, because various state quantities (such as pressure, temperature, and flow amount) in an industrial process must be controlled with high precision, real-time performance is important.